This invention relates to a waste heat recovery boiler for generating steam for driving a steam prime mover by utilizing a flue gas discharged from a gas turbine, and more particularly to a waste heat recovery boiler for abating an NO.sub.x concentration of the flue gas.
One of main problems encountered in the development of gas turbines is an abatement of NO.sub.x in flue gas. To this end, an apparatus for decreasing the NO.sub.x concentration in a combustion apparatus and removing NO.sub.x from the flue gas has been so far proposed.
One of the apparatuses for removing NO.sub.x from the flue gas is disclosed, for example, in U.S. Pat. No. 4,106,286, and the process for removing NO.sub.x used in the apparatus is called a dry catalytic reduction-decomposition process, where ammonia is injected into a flue gas and then the flue gas is passed through a reactor filled with an iron oxide-based catalyst to decompose NO.sub.x into harmless nitrogen and steam by reduction. It is known in the process for removing NO.sub.x that a percent NO.sub.x removal greatly depends upon a reaction temperature of catalyst bed, that is, a temperature of combustion gas passing through the catalyst bed of the apparatus for removing NO.sub.x, as shown in FIG. 1. That is, as is evident from FIG. 1, the percent NO.sub.x removal is rapidly increased, as the reaction temperature increases from 200.degree. C. to 300.degree. C., and approaches almost a maximum above 330.degree. C., and almost the maximum percent NO.sub.x removal can be kept up to about 400.degree. C. The percent NO.sub.x removal is then lowered above 450.degree. C. Thus, it is desirable that the temperature of flue gas passing through the catalyst bed is 300.degree. to 450.degree. C., and the apparatus for removing NO.sub.x is provided between the evaporator and the economizer, where the temperature of flue gas can be obtained, in view of a rated load operation.
At a partial load operation, for example, 30% partial load operation, of the apparatus, the flue gas from the gas turbine has a temperature of about 250.degree. C., when it passes through the apparatus for removing NO.sub.x, and as is evident from the temperature characteristics of the apparatus for removing NO.sub.x shown in FIG. 1, the percent NO.sub.x removal is too much lowered to satisfy the desired abatement of NO.sub.x concentration.